Impaction Colic: Blocking the Way

There are moments spent with our horses that are often taken for granted: The welcoming whicker as you approach with dinner, the snuffling in the feeder for tasty bits and pieces, the melodic sound of chewing as your horse enjoys every morsel, and the sweet smell of hay as it is crushed between your horse's teeth. When these ritual sounds and senses of the day go missing, there is a sense of dread.

Your horse isn't interested in coming in for dinner. Out in the paddock, he is occasionally pawing the ground, half-heartedly. With a sigh, he plops to the ground, lying quietly. After a few minutes, that doesn't seem to suit him either. He rises, shakes his neck and head as if to throw off a cloak of discomfort, then stands despondently.

Although this presentation might be typical of a horse that simply isn't feeling well, it is also quite typical of a horse with intestinal discomfort from impaction colic.

What is Impaction Colic?

Impaction colic is caused by a blockage that forms due to feed material obstruction in the large colon. Nat White, DVM, MS, Dipl. ACVS, and Marco Lopes, DVM, MS, professor at the veterinary school in Brazil, have pursued colic research at the Marion duPont Scott Equine Medical Center in Virginia with support of the Grayson-Jockey Club Research Foundation. White, director of the Equine Medical Center, says, "Large colon impactions make up as much as 8-10% of all colic, but the cause in a large majority of the cases is not known. Nearly 30% of all equine colic cases at referral hospitals are attributable to an impaction."

As food traverses the digestive tract, it enters several segments of large colon that narrow considerably before opening into a somewhat wider portion of intestine. These narrow spots are more prone to blockages. Impaction can also develop in the cecum (less than 5% of all impactions) or in a portion of the small intestine leading into the cecum called the ileum.

An important function of the large colon is to provide a reservoir from which fluids and electrolytes are absorbed. Normally, 30-40 gallons of fluid are secreted daily into the upper bowel, with about 90% of this efficiently reabsorbed in the cecum and large colon. High-fiber diets composed of good-quality hay have the advantage of increasing colonic water by at least 30% over diets that are comprised of added grain products.

Motility of the large colon is best stimulated by the volume of food and water introduced to the intestines. Delay of movement through the colon causes more water to be absorbed out of the fecal contents.

Lopes and colleagues' study on gastrointestinal motility showed that, "Feed intake is a major stimulus for gastrointestinal motility, thus fasting leads to GI hypomotility (slow movement). Although fasting is unavoidable or even desirable in some situations, management practices to offer the opportunity for the horse to have a more continuous eating pattern best mimic natural conditions. Dehydration and electrolyte imbalances may also lead to GI hypomotility."

Dehydration causes drying of the colonic contents, and coupled with sluggish gut motility, there is potential to develop an impaction. Dehydration occurs subsequent to protracted exercise, intense sweating, inadequate water consumption, or hormonal changes. It can also occur when a horse doesn't drink enough due to frozen or polluted or contaminated water. Lopes says, "Preventing dehydration and electrolyte imbalance by providing continuous access to fresh water and feed (including a salt block) is indicated."

Effects of Feeding Strategies

Horses evolved to graze small amounts of fiber-rich plants for 13-15 hours a day, and this style of eating avoids overfilling of the stomach. In today's fast-paced society, horse-keeping must suit space and schedules. Not all horses have access to pasture, and not all horse owners can be present to feed small meals throughout the day.

Feeding large meals twice a day, particularly of grain, has profound effects on intestinal function. In a recent project by Lopes and White, the amount of water in the horse's colon was compared when feeding a hay diet or a diet of hay and grain.

"We hypothesized that increasing the grain would decrease the amount of water in the colon, perhaps creating the conditions for impaction," says White. Indeed, this is what was discovered, and it corroborated previous research findings: Horses fed a large grain meal twice a day experience a 15% reduction in plasma volume within 30 minutes of the meal. There is no change in plasma volume in horses fed smaller amounts of grain every few hours.

White elaborates, "Feeding grain decreases the amount of fiber in the diet, which decreases the water content of the colon and alters fermentation to produce more gas. The changes created by twice daily feedings of grain set up conditions for extremes in fluid exchange in the colon, which can dehydrate the ingested feed and potentially set up conditions that could lead to simple or severe colic."

For every pound of feed ingested, a horse needs two to four pints of water for digestion. This means a 1,000-pound horse consuming 20 pounds of food each day needs a minimum of 7.5 gallons (30 liters) of water to process the feed. This is the amount necessary solely for intestinal function; additional water is necessary for maintenance functions. Note that pelleted roughage requires more water for digestion, and it can to lead to impaction if water intake is restricted for any reason.

Dehydration of colonic contents is not the whole story. There are other important mechanisms that contribute to intestinal transit, such as viscosity of the food, diameter of the tube through which ingesta flows, and pressures within the intestine that progressively propel material towards the rectum while retaining them long enough to extract nutrients. In addition, offering a large meal only twice daily can interfere with the activity of intestinal microbes responsible for efficient digestion of fiber because of the change in pH that takes place when a horse digests food.

Overfeeding of indigestible material such as poor-quality hay is thought to be a significant contributing factor to the development of an impaction. "To prevent formation of ingesta that is too viscous (thick), it is better to avoid mature grass or any feed composed of poorly digestible fiber," says Lopes. "Whenever hay or grass is fed, the contents of the large intestine have two distinct phases: A solid phase, composed of undigested pieces of grass, and a liquid phase. This liquid phase is loose and can be moved independently from the solid phase, leaving behind a mass of solid particles that can form an impaction. If poor-quality roughage is fed, the properties of the solid phase change (i.e., larger particle size, increased particle stiffness) in a way that makes formation of a hard mass (an impaction) even more likely.

"Although not proven, there is evidence that poor dentition could also increase the risk of impaction by compromising mastication, which would lead to a situation similar to what is seen with ingestion of poor-quality roughage: Ingesta would be composed of larger particles. Thus, regular dental care is recommended," adds Lopes.

Lifestyle Effects

"Alterations in motility, water content within the colon, and changes in exercise are all considered as possible risk factors," says White of colic. "The problem appears more common in mares and middle-age horses. Events associated with the impaction include management changes within two weeks prior to signs. One of the most frequently reported management changes is sudden restriction of exercise due to a musculoskeletal injury, including hospitalization or restriction of exercise after surgery."

One study reported recent stall confinement was associated with 53.7% of impaction colic cases; another study found 62.5% of colon impactions occurred within two weeks of significant management changes, such as stall confinement or transport.

Lopes comments, "Ingestion of indigestible non-feed material such as shavings, sand, gravel, hard seeds (e.g., cockspur hawthorn, mesquite beans), hair, pieces of rope or twine, and pieces of rubber can also lead to GI impaction. Although most horses would not eat these materials, some may develop vices due to conditions such as prolonged stall confinement or hunger. Thus, the time horses are kept in stalls should be minimized."

White mentions concerns about diets fed to inactive horses: "Ideally, horses confined to a stall should not have any grain, as energy is rarely needed. Unfortunately, most owners or managers are not aware of the quality of hay they feed in terms of its energy and protein content." Hay should be analyzed for nutrients prior to being fed to the horse so a proper diet can be formulated to meet his energy requirements.

"During confinement or transport, use of bran mashes or other diet alteration has not been proven to prevent impactions, yet feed modifications add the risk associated with diet changes, which have been incriminated in causing colic," White adds.

Exercise provides multiple benefits by increasing metabolism and improving intestinal motility. White says, "Ad lib exercise is best for horses, as some exercise appears to favor the normal digestive process with frequent intake of small amounts of feed over time. Just as changing the diet can be a challenge to some horses' digestion, horses with acute decreases in activity should be monitored closely for digestive problems that can lead to colic."

Lopes concurs: "There is evidence light physical activity (i.e., walking) stimulates GI motility, so it is better to avoid maintaining a horse in a stall for long periods."

Fiber digestibility increases by up to 20% in exercised horses, promoting greater retention of the fluid part of the diet and shortened retention of the more formed, particulate part of the feed. Progressive movement of particulate materials down the intestinal tract promotes efficient digestion while not allowing it to linger to form dehydrated intestinal contents.

Feeding in the period surrounding exercise is not without problems. Rigorous exercise just prior to feeding can decrease feed digestibility while blood remains shunted to working muscles and away from the intestinal tract. White notes, "Strenuous exercise will shut down intestinal motility. Therefore, it makes sense not to have a large quantity of feed in the stomach or small intestine during exercise. Feeding forage at intervals during moderate or light exercise appears to be appropriate."

Proper measures should be taken to adequately cool out a hot horse before feeding large meals, particularly grain.

Other Causes of Impaction Colic

Sand ingestion also has the potential to create an impaction, particularly in the right dorsal colon. One of the best means of moving sand through the bowel is by promotion of active intestinal motility. Feeding a fiber-based diet that consists of at least 50% hay, roughage, and/or pasture keeps the intestines moving well and should encourage passage of small amounts of sand so it does not accumulate in the bowel. In some areas, psyllium is fed to help manage sand ingestion.

Coastal Bermuda grass (Cynodon spp.) common to the southeastern United States is known to elicit intestinal contractions around a mass of feed, causing more water to be compressed out of the material; this can lead to increased desiccation and firmness of the intestinal contents. Mature Coastal Bermuda grass has a high, non-digestible crude fiber content that increases its propensity to ileal impaction colic.

Tapeworm infestation has also been incriminated as a cause of ileal impaction. Lopes stresses that ivermectin does not affect tapeworms, so choose dewormers that specifically target tapeworms, such as a product that contains praziquantel.

Certain medications, such as phenylbutazone (Bute) and flunixin meglumine (Banamine), are often given to a horse for an injury or to reduce post-operative pain and swelling. Such non-steroidal anti-inflammatory drugs (NSAIDs) can contribute to the risk of cecal impaction by diminishing smooth muscle contractility (and thus motility) of the colon. Caution needs to be taken during a colic crisis to administer as little a dose of an NSAID as necessary so it will not mask clinical signs of a surgical colic condition.

It is this author's experience that significant orthopedic pain from hind limb injury or surgery is a risk factor in developing a cecal impaction. Lopes says, "The activity of the sympathetic nervous system contributes to reduce GI motility. Thus, conditions (such as pain) that activate the sympathetic nervous system should be controlled."

Signs of Impaction Colic

Transit of feed through the large colon takes several days, so what passes as manure today was ingested at least three days ago. An impaction might take days to form and to reveal itself as a problem. It is not until a horse feels abdominal tension that he will show discomfort.

When pain does develop, it usually starts out as mild and intermittent. An affected horse might at first appear depressed. He might look at his flank, paw, stretch, kick at his belly, exhibit a flehmen response (curling the upper lip), or spend a bit of time lying down. He might still nibble at food, but he seems finicky. Manure production is scant or absent. Feces that pass are dry and diminishing in quantity. The feces of a stagnant bowel might be coated with mucus and fibrin strands in the body's attempt to lubricate dehydrated fecal material. A normal horse will pass eight to 12 bowel movements in a 24-hour period; it is helpful to know what is normal for your horse, as this varies among individuals.

With a simple obstruction that's detected early, the horse's heart rate, temperature, and other vital signs are usually within normal limits, but the respiratory rate might accelerate relative to the degree of pain. The body's attempt to correct the blockage can create a hypermotile gut with active and noisy intestinal sounds in an attempt to squeeze the impaction through the bowel. These sounds can be misleading since not all intestinal activity results in progressive movement of material.

As a horse's status deteriorates with an unrelieved impaction, there is increasing distention and pressure on the bowel, and lessened blood flow to (and degeneration of) the intestinal lining. White suspects a link between colon displacement and a preceding impaction: "At surgery, many displacements have a colon filled with ingesta. Some appear to be impactions and some appear as increased ingesta."

The degree and frequency of pain will continue to worsen over time in a surgical case. Intestinal sounds and activity might cease (called ileus), which is dangerous.

Effective Treatment

"The basic premise for treating colon impaction is relief of pain, softening the consistency of the impacted ingesta, and stimulating motility to increase fecal transit," says White

An effective strategy uses overhydration via an IV. Overloading the blood vascular system with fluid increases secretion of intestinal water into the impaction to soften it, while also maintaining whole-body hydration and circulation to intestinal blood flow. IV fluid treatment might require 40-80 liters per day. Once the horse is hydrated, administration of a balanced electrolyte solution by stomach tube at a rate of 5-10 liters per hour helps hydrate colon contents and has been effective in treating horses with impactions.

Pain-relieving medications elicit smooth muscle relaxation of the intestinal wall, thereby minimizing spasms that tighten the intestine around an impaction and allowing gas and fluid to pass. A trailer ride or walking can be beneficial to evacuate gas and to stimulate intestinal motility.

It is important to stop a horse's food intake; more food might add to the size of an impaction. It might be necessary to apply a muzzle to prevent eating as he starts to feel better. White says, "Research suggests that colon hydration is increased when fasted horses are fed hay. Although feeding can increase motility and oral water intake, feeding horses with impactions should be delayed until there is evidence that the impaction is moving or is resolved. When initiating feeding after the impaction has moved out of the colon, a laxative diet without grain, such as grass or alfalfa hay, is preferred. Use of bran as a laxative should be avoided. Although pure bran is high in fiber, most milled bran contains large amounts of carbohydrate, which reduces total fiber content and potentially decreases colon water content."

Short periods of grazing might be helpful for its laxative action and high water content. Avoid overconsumption of grass as it is highly fermentable and might contribute to gas distention of the bowel.

The majority of impaction colics respond to medical treatment. Although called a simple obstruction, some impactions might not be resolved even with intense medical management or IV fluid therapy. Ongoing deterioration can necessitate surgical intervention due to a concurrent problem such as displacement of the intestinal loops or compromised bowel circulation. Obstruction of the large colon that persists for more than 24 hours might have an adverse effect on intestinal nerves, predisposing a horse to future impaction colic from a lack of nerve stimulation of muscles along the gastrointestinal tract. Rapid identification and resolution of an impaction is key to a successful outcome.

"Optimally, horses should be fed a diet with a minimum of 60-70% forage," says White. "This means feeding only pasture or hay, and only feeding grain when necessary. High-quality hay will provide adequate protein for most horses. The horse's intestine doesn't always tolerate acute changes in diet, so when a change is necessary, the transition should be made over time, normally seven to 10 days."

About the Author

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado,
and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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